1. Field of the Invention
This invention relates to the field of determining printing misregistration for printers, and more specifically, determining misregistration for each printing station in a printer having multiple printing stations.
2. Description of the Related Art
In order to ensure the best possible print quality from a printer, an operator typically will recalibrate the printer each day. Two of the most important factors that should be calibrated are density and registration. To calibrate the density, the operator will print out a control strip of the different colors (toners) used by the printer. The operator will then use a densitometer to measure the density of each of the patches in the control strip. Then, as a result of the density measurement reading of each of these patches, the operator will make some manual adjustments to the printer to correct for density in the different density ranges. Since maintaining optical density is a very important parameter that the operator needs to control for a printer, most operators have a densitometer for measuring the density.
Also, in a printing process, it is very important that all of the colors are correctly registered since colors are overlaid on top of each other to achieve a desired color as required in the subtractive printing process. If the colors are out of registration, a combination of colors will not lie directly on top of each other, and the quality of the color printing is distorted. In summary, maintaining accurate registration is required to get good color fidelity.
In order to calibrate the printer for registration, the printer will also print out a pattern of very small intersecting lines as shown in FIG. 1. The patterns are used for the registration of each color of the printer, e.g., cyan, magenta, and yellow, relative to black. The patterns are printed in both the X-direction and the Y-direction to calibrate the registration in both of these directions. For a printer having three colors, e.g., cyan, magenta, and yellow, six targets will be printed: three horizontal and three vertical. A greatly magnified illustration of these targets is shown in FIG. 1.
Typically, one can only determine the point of intersection with the aid of an eye loop or by using expensive and sophisticated imaging equipment. Ideally, the line should intersect at 0. If the line intersects at +1, the operator has to adjust the registration of the printer to make a correction for +1. Using an eye loop involves a "manual" process of eyeballing whether registration is off by examining the targets of colors that are laid on top of one another. With the aid of an eye loop, it may be difficult to readily determine whether the registration is +1, +2, 0, -1, or -2. Typically, experienced operators can only achieve a registration accuracy within +1 or -1 pel. Basically, these patterns are very difficult to read. For example, typically all of the patterns together, shown in FIG. 1, would take up an area of only about 0.5.times.0.25 inches.
As a typical scenario, an operator will, on a daily basis, use a densitometer to measure printed patches for adjusting the optical density of the printer, and will use an eye loop to measure another group of patterns for adjusting the registration. The patterns, i.e., targets, are typical of what are used in the industry to measure registration and the eyeball is the measuring device. Consequently, this registration process is totally nonautomated.
Although there are a variety of ways for determining misregistration, most of the currently known techniques are similar in that they all compare a line or dot to a reference line, as just described. Then, corrections are made based upon a difference in location with respect to an ideal value or reference distance. The measurement of this variation from the reference distance can be done in an automated manner via sophisticated and expensive image analyzer equipment, or done in a nonautomated manner via the use of a microscope or the equivalent. It may not be cost justified to use expensive image analyzer equipment for determining misregistrations in a relatively low-cost printer. On the other hand, the inaccuracies and lack of ease of use of the manual determinations may not be sufficient, either.